It is known in the art to provide a wheelchair having an automatic reclining backrest. It is also known in the art to provide a backrest which slides relative to a frame as the seat reclines and returns to its upright position, to reduce back shear on a user. An example of such a prior art device is described in U.S. Letters Pat. No. 5,556,157, issued to Wempe.
One drawback associated with such prior art devices such as Wempe is the inability to adjust the ratio of backrest slide relative to the back frame to account for larger or smaller users. In very large users, the pivot point between the legs and torso is positioned further away from the pivot point between the seat and the backrest. Accordingly, for such users it is desirable to have the backrest travel a large amount relative to the back frame as the seat is reclined. Conversely, for thin users, the pivot point between the user's legs and torso is closer to the pivot point between the seat and frame. In such cases, it would be desirable to provide for less sliding of the backrest relative to the back frame. Prior art devices, such as Wempe, use a single cable connection between the backrest and seat. This type of arrangement makes it difficult to adjust back shear between large and thin users.
An additional drawback associated with the prior art is the lack of means for sliding the backrest upward relative to the back frame as the seat is returned to its upright position. Devices such as that described in Wempe use a flexible cable connected between the seat and back frame to reduce back shear. While this assembly is useful to pull the backrest downward relative to the back frame, the resilient connection provides little upward pressure on the backrest relative to the back frame when the seat is returned to its upright position. Accordingly, the backrest is moved upward relative to the back frame only by the force of the user's back against the backrest, a force often insufficient to return the backrest to the full desired position. Accordingly, the seat reclines and returns to its upright position, the user becomes more and more slumped in the seat. It would be, therefore, desirable to provide means for fully raising the backrest relative to the back frame as the seat is moved into its full upright position
It is also known in the art to provide independently controlled electric motors to move the backrest relative to the back frame. Such an assembly has several drawbacks. One drawback associated with such an assembly is the cost not only for initial assembly, but also for maintenance and repair. A second drawback associated with such an assembly is the tendency for such devices to move out of synchronization with the reclining and upright movement of the backrest. As such devices move out of synchronization, the backrest starts to move either too much or too little relative to the back frame, and may even tend to move the backrest in an undesired direction relative to the back frame, during the initial or final stages of the recline. Accordingly, it would be desirable to provide means for maintaining synchronization of the backrest relative to the back frame. The difficulties encountered in the prior art discussed herein above are substantially eliminated by the present invention.